﻿312 PI.AGIOI.A 



with a wide, tliickcncd. double cd|j[e. which is sonictimcs cre- 

 nale ; branchial openiiif!^ larf^^e, f rinj:jed ; anal o])enin<^ smooth 

 or with only the faintest crenulations. 



'J'vpe, Lhiio cyrcnoidcs Philippi. 



The name Ptychodenna was published b\- me in the Synojjsis 

 Oct. 6, U)oo; von Martens liad ])ul)lished the name .Irtouaias 

 for practically the same .^rouj) in the iiiologia Centrali Ameri- 

 cana in February of the same year, but at the time my name 

 went throutili the press I had not seen it. 



{'irou]) of Plai:;i(>la cyrcnoidcs. 



Shell ireneralh' more or less covered with radiatinj;' grooves, 

 which cut up the concentric scul])ture into loo])ed wrinkles or 

 nodules, sometimes having' the appearance of dried, wrinkled 

 l)aint, the sculpture extending into the substance of the shell; 

 epidermis usually tawny or yellowish, rarely showing any 

 vestiges of rays : female shell much jiroduced in the j)Ost-basal 

 region. 



Pi.AC.ioL,A cvRENOiDRS (Philippi). 



Shell solid, somewhat inflated, subtriangular or subquad- 

 rate, with rather full and (piite higii beaks, whose sculpture 

 consists of numerous fine, douibly-looped, corrugated ridges, 

 wdiich gradually fade into the regular shell sculpture ; posterior 

 end obliquely truncate above, with a rounded point about mid- 

 wav up from the base : posterior ridge well developed ; base 

 well rounded ; surface sculptured with slightly corrugated, 

 concentric ridges ; epidermis uniform tawny, tawny-green or 

 brownish, cut with impressed radial lines, especially behind, 

 often looped, wrinkled and granulous ; left valve with two rag- 

 g^ed pseudocardinals, the anterior one somewhat compressed, 

 and two curved laterals, the lower higher; right valve with 

 two pseudocardinals, the upj^ier feeble and compressed, and one 

 lateral ; beak cavities shallow ; muscle scars im])ressed : nacre 

 whitish, often lurid or blotched, thinner behind. Male shell 

 with a decided rounded point behind scarcely midway up 

 from the base, base evenlv rounded. Female shell with a 



